Aircraft wings generally include so-called high lift devices extending from the leading and/or trailing edges of the wing to increase aerodynamic lift during takeoff and landing of the aircraft. When extended from the wing, the high lift devices increase the effective size, curvature, camber and area of the wing, thereby increasing the lift of the wings for slow speed flight. High lift devices extending from the leading edge of the wing are commonly known as slats. Those extending from the trailing edge of the wing are commonly known as flaps.
The aircraft control causes movement of the flaps and slats by driving actuators, which may be hydraulic or pneumatic or electrical actuators.
Various situations can lead to asymmetry between the flaps or slats on the wings. The flaps or slats on one wing can, for some reason, extend or retract more than the flaps or slats on the other wing or flaps or slats on one wing can become jammed in position. Flaps or slats or other panels or components on the same wing/other parts of an aircraft can also exhibit undesirable asymmetry.
Excessive flap or slat asymmetry between wings can be catastrophic. Asymmetry between flaps can cause an uncommanded and pronounced roll towards the wing with the lesser amount of flap extended. Excessive asymmetry between slats can cause uncommanded yaw. This can result in a spin or other loss of control of the aircraft. Furthermore, such asymmetry can cause one wing to produce more lift than the other wing resulting in a stall situation.
Many aircraft manufacturers have installed provisions for detecting and minimising an asymmetric flap or slat condition. In a simple approach, the cockpit displays include needles indicating the positions of the slats/flaps. So long as the needles overlap, the flaps or slats are considered to be symmetrical. Should the two needles separate, the pilot becomes aware that an asymmetric flap or slat situation is occurring and the pilot must intervene to stop movement of the flap or slat.
More sophisticated systems incorporate position sensors and a brake system. If the position sensors detect an asymmetric situation, brakes are automatically activated to stop further movement of the flaps or slats, thus preventing the situation from becoming worse.
Asymmetry can develop very rapidly in the event of mechanical transmission failures and, therefore, high speed monitoring is required to safely arrest or brake disconnected sections of the high lift system to prevent asymmetry.
Asymmetry in other components can also cause problems. The invention is described below in relation to asymmetry between flaps or slats on different wings, but is not so limited. The invention can also be used to respond to asymmetry between flaps or slats or any other components on the same or different wings or, indeed, on different system or aircraft parts. The invention can be used wherever it is necessary to respond to asymmetry between two components.
Typical asymmetry monitors compare wingtip position sensors at high rate (˜every 2-3 milliseconds) and calculate the measured asymmetry between wings. This measurement is compared to an asymmetry threshold, and if the threshold is exceeded a fault flag is set “true”. The fault flag is passed to a fault integrator which establishes whether the fault is transient (in which case it may simply be due to sensor noise) or persistent (as would be the case for a true asymmetry). If the fault integrator confirms the fault as a true asymmetry it then sends command signals to brakes within the system to arrest the transmission, hopefully before the level of asymmetry has grown beyond the safe limit.
There is, therefore, a delay (typically 20-30 milliseconds) between identifying the asymmetry and operating the brakes, whilst the integration step is carried out.
This delay was intentional, to prevent spurious application of the brakes, in the event of a signal indicating asymmetry which could, for example, have been caused by signal noise, rather than a “true” asymmetrical situation. In the conventional systems, the indication of an error has been confirmed to be persistent for a given number of samples before any action is taken in respect of the brakes.
The aim of the present invention is to provide an improved asymmetry monitoring system and method that provides a more rapid response, but still provides a check against response to spurious signals.